Synthesis of Macrocyclic Polyazomethines

The synthesis of 20-membered macrocyclic polyazomethine dyes 3a and 3b from 4,4'-dihydroxybiphenyl-3,3'-dicarbaldehyde (2a) and 5,5'-diamino-4,4'-dihydroxybiphenyl-3,3'-dicarbaldehyde (2b) and macrocyclic polyazomethine pigments 4 and 5 from 3a is described.


Introduction
Azomethines are used as substrates to prepare a large number of industrial and biologically active compounds via ring closure, cycloaddition and condensation reactions between primary amines and aldehydes. 1Moreover, azomethines are also known to have biological activities such as antimicrobial, [2][3][4][5] antifungal, 4 6 antitumour [7][8][9] and as herbicides. 10In industry, they have a wide range of application such as dyes and pigments with luminescent properties. 113][14][15][16][17] For that reason, a huge number of monoand bis-azomethines are described in the articles.][20][21] On the other hand, macrocyclic compounds like crown ethers and cryptands have received much attention in the last few years both in chemistry and in biology.Koriakin and Angelov 22 prepared two azomethines bearing crown ether moiety with the aim of the synthesis of metalloenzymes.
Thus the synthesis of macrocyclic azomethines, bearing properties of both azomethines and macrocyclic compounds, is very interesting.In the current paper, four novel 20membered cyclic polyazomethines have been synthesized.

General
All of the chemicals used were of commercial grade and were further purified by recrystallisation and redistilled before use.The solvents used were spectroscopic grade. 1 H NMR spectra were obtained by use of BRUKER WM-300 (300MHz).Standard pulse sequences were used for the NMR experiments.Chemical shifts ( ) are given in ppm and coupling constants (J) in Hz.The IR spectra were obtained on a Thermo Nikolet spectrometer scanning between 4,000-400 cm -1 using KBr pellets.UV-Vis absorption spectra were measured in CF-26 spectrometer (produced in Russia).Elemental analyses were performed using Heraeus CHNO-Rapid analyzer.Melting points were determined by Electrothermal 9100.Hydrazine hydrate was prepared form hydrazine sulfate and ammonia as described earlier. 18or molecular modeling studies, structures were generated with the aid of Chem3D Ultra-9.00 and HyperChem-v.6.02 software.Lone pairs of electrons and hydrogen atoms were added where appropriated.The equilibrium geometries of compounds were located using MM+ (for HyperChem) and MM2 (for Chem3D) functional set.In the next step, RHF calculation (semiempirical AM1 method) were performed and bond length, angles, torsion angles and partial charges have been calculated.Calculations were performed on a 750 MHz Pentium III computer with 256 RAM.

General procedure of preparation of aldehydes 2a and 2b
To a stirred solution of 1a or 1b (35 mmol) in ethanol (150 mL) was added the solution of sodium hydroxide (3.2 g, 80 mmol) in water (20 mL).The reaction mixture was heated up to 80 °C.Then, chloroform (9 mL, 0.11 mol) was added dropwise and the reaction mixture was boiled for a period of 1 hour.The excess of ethanol and chloroform was distilled.Hydrochloric acid was added subsequently until pH 5-6.The residue was dissolved in the minimal amount of diethyl ether and equal amount of saturated solution of Na 2 S 2 O 5 was added.The mixture was kept for 24 hours and the precipitated solids were filtered off.The solid was dissolved in water and treated with sulfuric acid (10%).The yellow crystals were obtained by filtration and dried on air.

General procedure of preparation of macrocyclic polyazomethines 3a and 3b
Into the four-necked flask fitted with two dropping funnels, mechanical stirrer and reflux condenser 2-propanol (100 mL) was placed and heated until boiling.The solution of 2a or 2b (1.0 mmol) in 2-propanol (10 mL) and freshly obtained hydrazine hydrate (2.2 mmol, 50%) were added from different dropping funnels simultaneously for a period of 30 min.The reaction mixture had been heated for 1 hour and precipitated pale yellow crystals were isolated.

Polyazomethine 4
To a solution on the macrocyclic polyazomethine 3a (1.8 g, 3.78 mmol) in ethanol (60 mL) stirred at 30 °C was added bromine (22.68 mmol) dropwise.The temperature was maintained at 30-32 °C for a period of 45 min after complete addition of bromine.The reaction mixture was cooled up to room temperature and filtered off.The solid was washed with hot water and ethanol.(
Aldehydes 2a,b have been prepared under various conditions.The successful result has been obtained when reaction has been carried out in ethanol under Reimer-Tiemann conditions.The crude products 2a,b are lightbrown solids.The multiple recrystallizations in the various solvents or solvent mixtures do not result in purification of product.The satisfactory purity may be achieved by purification via corresponding bisulphite derivatives.
The formation of carbonyl compounds 2a and 2b is confirmed by various methods.2a and 2b reacts with Tollens' reagent with formation of silver.IR spectrum shows absorption bands on 2820-2810 and 1705-1695 cm -1 , characterizing C-H (aldehyde) and C=O bonds, respectively.In addition the 1 H-NMR data are in good agreement with the above mentioned results.Namely, 1 H-NMR spectra of 2a showed only six aromatic protons, represented as two doublets and one singlet.The 1 H-NMR data of 2b involve signals of four aromatic protons.The appearance of singlet at 10.24 for 2a and at 10.02 for 2b indicates to formation of carbonyl compounds.
Condensation reactions between 2a,b and freshly synthesized hydrazine hydrate have been carried out under high-dilution.The reacting agents 2a,b as isopropanol solutions and hydrazine hydrate were added to isopropanol drop-wise simultaneously from two different droppingfunnels.Products 3a and 3b have been isolated, after boiling the reaction mixture for a period of approximately 1 h, as yellow crystals with the yield up to 53-58%.
The formation of desired products is established with UV-Vis, IR and 1 H-NMR spectroscopy.UV-Vis spectra of 3a and 3b contain multiple bands in the near-UV range, while starting compounds 2a and 2b have no absorption bands in this range (Figure 1).It should be noted, that multiple absorption bands are characteristic for azomethine compounds.The long wave absorption band of 3b undergoes bathochromic shift for 5 nm because of influence of conjugated substituents (amino groups).The above described observation has been confirmed by results of the CNDO/S semi-empirical studies of 3a and 3b.
In the IR spectrum of 3a the disappearance of absorption bands at 2820, 2765 and 1705 cm -1 and formation of new absorption band at 1373 cm -1 indicates the conversion of aldehyde group into azomethine one. 1 H-NMR spectrum of 3a displays five signals, represented as 2 doublets and 3 singlets.The absence of the low-field signal ( 10.24) and the new signal, that appears at 6.88 point to successful reaction.Similarly, IR spectrum of 3b contains absorption band at 1355 cm -1 and 1 H-NMR spectrum-chemical shift at 6.85, that corresponds to N=CH group.
Polyazomethine dye 3a may be converted into the interesting polyazomethine pigments 4 and 5 according to the Scheme 2.
The treatment of the ethanol solution of 3a with bromine at 30 °C gives macrocyclic polyazomethine pigment 4. The pigment 5 has been obtained according to Vilsmeier reaction.The solution of 3a in DMF under the action of POCl 3 yields yellow pigment 5. Unexpectedly, compounds 4 and 5 are insoluble in most organic and inorganic solvents.For example, all our attempts to dissolve these compounds into the solvents such as methanol, ethanol, isopropanol, toluene, benzene, xylenes, DMF, chloroform, hexane, ethyl acetate, concentrated sulfuric acid and water solution of sodium hydroxide was unsuccessful.For this reason, unfortunately, the spectral investigation has not been carried out.
The elemental analysis of 4 and 5 showed that proposed compounds contained four bromine atom and aldehyde group, respectively.In addition, the presence of aldehyde group is established by successful reaction with Tollen's reagent.
The location of the functional groups was suggested upon the base charge-distribution in 3a, calculated by AM1 semi-empirical method.The most negative charge is located on the ortho-carbon atom from carbon atom bonded to hydroxyl.
The macrocyclic polyazomethine dyes 3a and 3b as well as macrocyclic polyazomethine pigments 4 and 5 present well pronounced fluorescent property in solid state (magenta luminescence under UV-light exposure).

Quantum-chemical calculation
Interesting data were obtained from quantum-chemical calculations of these macrocyclic compounds.Optimized models of structures are rather "thin" molecules.The thickness of molecules does not exceed 3.56 Å. Hydrogen of the CH=N groups is oriented out of cycle hole.The macrocycle hole diameter is up to 4.2-4.33Å(Figure 2).We suggest, that such wide hole will allows to form stable complex compounds with ions of large size and this complexation is, currently, under investigation.Charge distribution among the cycle forming atoms vary from -0.138 to +0.138.The irregularity of charge distribution provides to generate dipole moment with the value of 3.352D and 1.977D for 3a and 3b respectively.The vector of dipole moment is positioned in the "crown" center and directed at right angle to plane of molecule.
It must be noted that the open chair azomethines have greater dipole moment than 3a and 3b.Additionally, introduction of strong electron-releasing amino groups causes dipole moment decrease.

Conclusion
In summary it has been described the method of the synthesis of 20-membered cyclic polyazomethines compounds according to two step conversion, containing carbonylation and condensation reaction with hydrazine hydrate.Both reactions may be carried out under rather simple routines with moderate yields.In addition, the synthesized macrocyclic polyazomethines may undergo various electrophilic substitution reactions, giving valuable macrocyclic polyazomethine pigments.
The desired products are characterized with well pronounced fluorescent properties in the solid state.Amino groups presented in 3b may be used for further conversions into novel macrocyclic azomethine compounds, which is under current study.